Wednesday, 28 May 2014

The PS Audio DirectStream DAC - Part I.

At BitPerfect we needed a reference-quality DSD-compatible DAC, and, without mentioning names, the ones we had to hand were proving not be up to the standards we were hoping for. To a certain extent, this is a self-fulfilling prophesy. At the end of the day the differences between DSD and PCM, while real enough, are actually quite small. On the amazing Da Vinci Dual DAC (which costs over $31,000) it certainly is subtle, although real enough that once you’ve heard it you want more of it. So at the sane end of the price spectrum, where should one be setting realistic expectations?

We found out, eventually, that we weren’t going to find what we needed at the truly sane end of the price spectrum, so we set our sights a little higher. Talking to Paul McGowan of PS Audio, we decided that their new DirectStream DAC, which was still in development at the time, was in all likelihood what we needed, so we committed to purchase one of the first production models. I have been listening to it for the last week. I’m listening to it as I type. I’ve been doing little else for the last week. I don’t normally do product reviews - I am not a professional reviewer - but I felt that in this instance the effort was probably justified. Please read on.

The PS Audio DirectStream DAC is not cheap. At $6,000 it is priced at a point which all of my friends would unhesitatingly label “insane”. But in the world of high-end audio, the market to which it is targeted, it has the potential to be classified as a bargain. Provided it delivers on the hype, that is. C’mon, you already know it does, because otherwise I wouldn’t be writing this! But let’s play along anyway, and pretend you haven’t already figured out that the butler did it.

The DirecstStream’s design eschews the conventional wisdom regarding the design of a high-end DAC, which says you go out and buy a chip-set from one of the established vendors such as ESS, TI, and Wolfson, and build yourself a DAC around it. Rather like you would build a computer, having chosen an appropriate CPU from Intel’s catalog. PS Audio has taken a different tack. They have avoided using a DAC chip entirely, and have built their entire core converter functionality around a FPGA which switches the output voltage between two seriously stable voltage levels, representing the ‘1’ and the ‘0’ of a DSD bitstream. This is quite a smart approach, but it requires every single input data stream to be converted to a 1-bit format, and the one they have chosen is 1-bit / 5.6448MHz, otherwise known as DSD128.

As it happens, the chip-based solutions don’t quite do it this way. Instead of a 1-bit format, they use a 3-bit, 4-bit, or even 5-bit format, and there is a good reason for doing that. The devilishly complex Sigma-Delta Modulators (SDMs) used to perform these conversions are fundamentally unstable when configured with a 1-bit output. This instability can be avoided entirely by using a multi-bit output format. But the actual D-to-A conversion using those multi-bit bitstreams is more complex. In effect, PS Audio has traded the electronic complexity of a multi-bit high-speed D-to-A converter for a design approach they can get their considerable electronic design chops around. All that needs to be done is to navigate a way around the 1-bit SDM instability problem. As it happens, there are various techniques to mitigate this problem, although it cannot be conclusively eliminated. But by taking the right measures it can be beaten into submission, otherwise SACD/DSD couldn’t be made to work.

Inside the DirectStream, all input formats - DSD128 included - are converted to a single 30-bit 10MHz format, and from there converted to DSD128 which is fed to the core converter. There are a couple reasons for this. The first is that digital volume control can be easily implemented in this intermediate stage. The second - behind which hide an advanced degree’s worth of technical matters - is that the analog output filters in the output stage assume a certain implementation of the DSD128 data stream to which an arbitrary incoming data stream might not adhere. [My thanks to Ted Smith of PS Audio for clearing that up for me.]

Paul McGowan has created a lot of hype around the DirectStream through his intriguing claim that it manages to extract more musical information from ordinary CDs (read 16/44.1-formatted music) than any other DAC. By and large, this claim seems to be backed up by other people who have heard the DirectStream for themselves. So a significant part of this review will be devoted to assessing its Red Book (16-bit, 44.1kHz PCM) performance, although don’t expect me to pronounce one way or the other on McGowan’s claims.

My test setup was as follows. The DirectStream was fed by a 2013 base-spec Mac Mini, running OS/X 10.9.3 with iTunes 11.2.1 and BitPerfect 2.0.2 (a pre-release beta). The DirectStream was fed into a Classé CP800 preamplifier which in turn fed a Classé CA2300 power amplifier. The loudspeakers were B&W 802 Diamonds. The Mac Mini used a PS Audio Jewel Power Cord, and the USB cable was a 1m Nordost Blue Heaven. All other Power Cords, plus all the (balanced) interconnects were BitPerfect Digital Precision products. The speaker cables were dual (bi-wired) runs of Cardas Golden Cross. Not only the DirectStream, but also its Power Cord and its interconnects, were all new and unused and so a period of break-in was required.

Powered up for the first time, and prior to any break-in, the sound was noticeably thin, brittle, and edgy. This could be down to the DirectStream or any of the brand new cables (and I have little interest in finding out which was responsible for what), but nevertheless you could tell immediately that here was something with serious detail in its sonic presentation. I set it up for a period of continuous break-in which I was prepared to last for at least a good week. In fact the improvement over the first 24 hours was quite dramatic, and by the end of the fourth day the previous 24-hour period seemed to have wrought no significant improvements, so at that point serious listening commenced. If significant break-in over a time frame of weeks should occur, I might report on it later.

PS Audio makes a point of recommending that you connect the DirectStream directly to the inputs of your Power Amplifier for a potentially ideal listening experience. It took me until the sixth day of listening before I got round to that, but the result was so immediately and obviously superior that this instantly became my preferred configuration. This entire review uses the DirectStream connected directly to the Power Amplifier’s inputs.

Before going on, there are a couple of matters specific to BitPerfect users. The first is that DirectStream only supports PCM sample rates up to 192kHz. However, in order to deliver DSD128 support, it must also support a PCM stream format at 352.8kHz which it duly announces to OS/X. However, if you send it a true 352.8kHz PCM data stream it does not play properly. Therefore it is important to set BitPerfect’s “Max Sample Rate” to 192kHz. I would also check “Upsample by Powers of Two” so that any DXD (24/352.8) tracks which you may have - and I have quite a few - will be downsampled to 176.4kHz rather than 192kHz.

The second matter relates to volume control, and requires an extended discussion. First of all I used BitPerfect’s Volume Control (the slider in the menu bar drop-down menu), which addresses the DAC’s USB-accessible volume control. While this does indeed control the volume it has some unexpected behaviour. The first is that it operates independently of the DirectStream’s own volume control, and cannot be addressed within the DirectStream itself. The second is that when used on a DSD track it causes the volume to be muted. BitPerfect Users used to using the volume control via its keyboard shortcuts may find this behaviour slightly annoying. According to Paul McGowan this behaviour arises because the XMOS USB receiver is unexpectedly acting upon the USB-delivered volume control commands. At this point, I am not sure which direction PS Audio will take when they get around to addressing this issue. Having a USB-accessible volume control is usually quite desirable in a Computer-based audio system, and I hope they will take that into account.

A personal irritation is that the DirectStream’s internal volume control is calibrated on a 0-100 scale, with 100 representing 0dB and each ‘1’ dialing in 0.5dB of attenuation (and ‘0’ being muted). I personally prefer to see the dB attenuation scale indicated (which is actually more common these days). In comparison, 0-100 is a bit like a car’s speedometer calibrated as 0-100%. “Honestly, officer, I was only doing 55%!!”. But I appreciate that other users may have a different perspective.

Build quality of the DirectStream is everything you would expect from a $6,000 component. Mine is finished in silver (black is an option), and has an unusual glossy piano-black top surface which looks very crisp, and as far as I can tell has a primarily cosmetic function. Sitting on top of my equipment rack it also ties in visually with my piano-black 802 Diamonds. For a DAC it is surprisingly heavy, not least because its analog output stage unusually uses a transformer (although I have not opened it up to see how big the transformer is). The heft also suggests a generously-specified linear power supply. Given PS Audio’s expertise in audio power supply, this would not be in the least surprising. The front panel is bare except for a small touch-panel display. This display can be dimmed if preferred (turned off, actually) using a button on the supplied remote control, but having done so I could not un-dim it without powering down the DirectStream. Am I being unfair by observing that the remote itself is a bit plasticky, in comparison with the hewn-from-a-solid-billet look and feel of the DirectStream itself? The similarly-priced Classé CP800 has a remote that might break your toe if you were to accidentally drop it. All things considered, the DirectStream’s physical presence is fully consistent with its price tag.